1. Field of the Invention
The present invention relates generally to asynchronous Wideband Code Division Multiple Access (WCDMA) communication. In particular, the present invention relates to a method and apparatus for signaling user equipment (UE) status information for uplink packet transmission in a soft handover region.
2. Description of the Related Art
A Universal Mobile Telecommunications Service (UMTS) system which is a 3rd generation mobile communication system that is based on Global System for Mobile Communications system (GSM) which is a European mobile communication system and uses Wideband Code Division Multiple Access (WCDMA), provides a consistent service capable of transmitting packet-based text, digitalized audio or video, and multimedia data at a high rate of 2 Mbps or higher no matter where mobile phone users or computer users are located. UMTS uses the concept of virtual access called “packet-switched access” that uses a packet protocol like Internet Protocol (IP) to access any terminal in the network.
FIG. 1 is a diagram illustrating a configuration of a conventional UMTS Terrestrial Radio Access Network (UTRAN). Referring to FIG. 1, a UTRAN 12 includes radio network controllers (RNCs) 16a and 16b, and Node Bs 18a, 18b, 18c and 18d, and connects a user equipment (UE) 20 to a core network 10. Each of the Node Bs 18a, 18b, 18c and 18d can have a plurality of cells in its lower layer. The RNCs 16a and 16b each control their associated Node Bs 18a, 18b, 18c and 18d in their lower layers. For example, in FIG. 1, the RNC 16a controls the Node Bs 18a and 18b, and the RNC 16b controls the Node Bs 18c and 18d. The Node Bs 18a, 18b, 18c and 18d each control their associated cells. One RNC and its associated Node Bs and cells controlled by the RNC constitute a radio network subsystem (RNS) 14a or 14b. 
Each of the RNCs 16a and 16b assigns or manages radio resources of its Node Bs 18a to 18d, and each of the Node Bs 18a to 18d provides the radio resources. The radio resources are generated per cell, and the radio resources provided by the Node Bs 18a to 18d refers to radio resources of cells managed by the Node Bs themselves. The UE 20 can create a radio channel using a radio resource provided by a particular cell of a particular Node B, and perform communication using the created radio channel. Because distinguishing between Node Bs 18a to 18d and their associated cells is meaningless to the UE 20 and the UE 20 recognizes only the physical layers created per cell, the terms “Node Bs 18a to 18d” and “cells” will be used herein interchangeably.
An interface between the UE 20 and RNCs 16a and 16b is called a Uu interface, and its detailed hierarchical structure is illustrated in FIG. 2.
FIG. 2 is a diagram illustrating a hierarchical structure representing an interface between a UE and an RNC. The Uu interface is divided into a control plane 30 used for control signal exchange between the UE 20 and the RNCs 16a and 16b and a user plane 32 used for actual data transmission.
Referring to FIG. 2, the control-plane (C-plane) 30 has a radio resource control (RRC) layer 34, a radio link control (RLC) layer 40, a media access control (MAC) layer 42, and a physical (PHY) layer 44, and the user-plane (U-plane) 32 has a packet data control protocol (PDCP) layer 36, a broadcast/multicast control (BMC) layer 38, the RLC layer 40, the MAC layer 42 and the PHY layer 44. Among the layers illustrated herein, the PHY layer 44 is located in each cell and the MAC layer 42 through the RRC layer 34 can be located in a RNC.
The PHY layer 44 provides an information transfer service using a radio transfer technique, and corresponds to Layer 1 (L1) of the Opening Systems Interconnection (OSI) model. Connection between the PHY layer 44 and the MAC layer 42 is achieved by transport channels, and the transport channels are defined according to how specific data is processed in the PHY layer 44.
The MAC layer 42 is connected to the RLC layer 40 through logical channels. The MAC layer 42 delivers data received through a logical channel from the RLC layer 40 to the PHY layer 44 through a proper transport channel, and delivers data received through a transport channel from the PHY layer 44 to the RLC layer 40 through a proper logical channel. In addition, the MAC layer 42 inserts additional information into data received through a logical channel or a transport channel, or analyzes additional information inserted into data and performs an appropriate operation according to the analyzed additional information. Further, the MAC layer 42 controls a random access operation. In the MAC layer 42, a part related to the user plane 30 is called MAC-d, and a part related to the control plane 32 is called MAC-c.
The RLC layer 40 manages setup and release of a logical channel. The RLC layer 40 can operate in one of three operation modes comprising an acknowledged mode (AM), an unacknowledged mode (UM) and a transparent mode (TM), and each operation mode provides a different function. Generally, the RLC layer 40 has a function of disassembling or assembling a service data unit (SDU) provided from an upper layer in an appropriate size, and an error correction function.
The PDCP layer 36 is located in an upper layer of the RLC layer 40 in the user plane 32, and has a function of compressing and decompressing a header of data transmitted in the form of an IP packet and a function of losslessly-transmitting data in a situation where a RNC providing a mobile service to a particular UE is changed.
A characteristic of the transport channels connecting the PHY layer 44 to its upper layers is determined by a transport format (TF) that defines physical layer processing processes, such as convolutional channel encoding, interleaving and service-specific rate matching.
A UMTS system uses an enhanced uplink dedicated channel (E-DCH) so as to enhance packet transmission performance in uplink communication from a UE to a Node B. In order to support stabilized high-speed data transmission, the E-DCH supports such techniques as Hybrid Automatic Retransmission Request (HARM) and Node B-controlled scheduling. In the MAC layer, a part managing processing of the E-DCH is called MAC-e.
FIG. 3 is a diagram illustrating a conventional method of transmitting data over an E-DCH in a radio uplink. Referring to FIG. 3, reference numeral 100 represents a Node B supporting the E-DCH, and reference numerals 101, 102, 103 and 104 represent UEs transmitting the E-DCH. The Node B 100 analyzes channel conditions of the UEs 101 through 104 that use the E-DCH, and schedules data rates of the UEs 101 through 104 according to the analysis result. In order to increase the entire system performance, the scheduling is performed in such a manner that UEs (e.g., UEs 103 and 104) located farther from the Node B 100 is assigned a lower data rate and UEs (e.g., UEs 101 and 102) located nearer to the Node B 100 is assigned a higher data rate as long as a measured Rise-over-Thermal (RoT) value of the Node B 100 does not exceed a target RoT value.
FIG. 4 is a signaling diagram illustrating a conventional procedure for transmitting and receiving messages over an E-DCH. Referring to FIG. 4, in step 202, a Node B and a UE set up an E-DCH therebetween. The E-DCH setup process 202 includes a process of transmitting messages through a dedicated transport channel. After the E-DCH setup, the UE provides UE status information to the Node B in step 204. The UE status information can include UE's transmission power information representing uplink channel information, information on available extra power of the UE, and the amount of transmission data piled in a UE's buffer.
In step 206, the Node B, which receives scheduling information from a plurality of UEs in communication with the Node B, monitors UE status information received from the plurality of UEs in order to schedule a data rate of each UE. In step 208, the Node B determines to grant the UE to transmit an uplink packet and transmits scheduling assignment information to the UE. The scheduling assignment information includes a granted maximum data rate and granted transmission timing.
In step 210, the UE determines a transport format (TF) of the E-DCH to be transmitted in a reverse direction, using the scheduling assignment information. The UE transmits uplink (UL) packet data over the E-DCH in step 212, and at the same time, transmits the TF information, i.e., a transport format resource indicator (TFRI), to the Node B in step 214. In step 216, the Node B determines whether there is an error in the TF information and the packet data. In step 218, the Node B transmits a non-acknowledge (NACK) to the UE over an ACK/NACK channel if there is an error in any of them. However, if there is no error in both of them, the Node B transmits an acknowledge (ACK) to the UE through the ACK/NACK channel.
If the ACK is transmitted indicating the completed transmission of the corresponding packet data, the UE transmits new data through the E-DCH. However, if the NACK is transmitted indicating the transmission error of the corresponding packet data, the UE retransmits the same packet data over the E-DCH.
The E-DCH, as it is an upgraded dedicated channel (DCH) for packet transmission of the transport channel, has the basic characteristics of the dedicated channel, and one of the characteristics is to support soft handover. When the soft handover is supported, a UE located in a soft handover region can set up E-DCHs to all of Node Bs included in its active set.
FIG. 5 is a diagram illustrating a conventional operation for supporting soft handover for an E-DCH. Referring to FIG. 5, a UE 504 includes Node Bs 501, 502 and 503 in its active set. In uplink power control, the UE 504 creates one combined transmit power control command (TPC) by combining a TPC#1 506 transmitted from the Node B#1 501, a TPC#2 507 transmitted from the Node B#2 502, and a TPC#3 508 transmitted from the Node B#3 503, and determines transmission power for uplink transmission of E-DCH data 505 depending on the combined TPC. According to the conventional TPC combining method, the UE 504 decreases transmission power of the E-DCH 505 by a predetermined value if any one of the TPCs 506, 507 and 508 is a DOWN command, and increases the transmission power of the E-DCH 505 by a predetermined value if all of the TPCs 506, 507 and 508 are UP commands. This method is called an “OR-of-DOWN method.”
The UE 504 in soft handover performs a HARQ operation in the following manner. The UE 504, after transmitting the E-DCH data 505, receives ACKs/NACKs 511, 512 and 513 from the Node Bs 501, 502 and 503, respectively. If any one of the ACKs/NACKs is an ACK signal, the UE 504 ends the HARQ operation, i.e., a retransmission operation, on the current E-DCH data 505. However, if all of the ACKs/NACKs 511, 512 and 513 are NACK signals, the UE 504 retransmits the same E-DCH data 505.
That is, if only the Node B#1 501 receive the E-DCH data 505 transmitted by the UE 504 without error and the other Node Bs 502 and 503 fail to normally receive the E-DCH data 505 transmitted by the UE 504, a RNC 510 to which the Node Bs 501, 502 and 503 are connected can correctly receive information included in the E-DCH data 505 transmitted by the UE 504. Therefore, if only one of the Node Bs 501, 502 and 503 included in the active set succeeds in receiving the E-DCH data 505, the HARQ retransmission is no longer required.
The UE located in the soft handover region simultaneously receives scheduling assignment information related to the E-DCH from several Node Bs included in the active set. Among the Node Bs included in the active set, a Node B having the best condition for scheduling the UE is selected as a best scheduling Node B (that is, serving Node B), and the other Node Bs are selected as non-best scheduling Node Bs (that is, non-serving Node Bs). Non-serving Node Bs refer to Node Bs that are included in the active set of the UE but have failed to be selected as the serving Node B. Compared with the non-serving Node Bs, the serving Node B has a higher authority in scheduling the UE located in the soft handover region. The UE determines a transport format (data rate, coding rate, etc.) of the E-DCH to be transmitted in the uplink direction by combining scheduling assignment information from the serving Node B with scheduling assignment information from the non-serving Node Bs.
While a scheduling method of the serving Node B is used at the same ratio as the method used for scheduling UEs located in a non-soft handover region, scheduling of the non-serving Node B is performed in a passive method for minimizing interference to other Node Bs included in the active set. That is, compared with the scheduling assignment information of the non-serving Node B, the scheduling assignment information of the serving Node B becomes a greater factor in determining an E-DCH by the UE.
However, the UE located in the soft handover region undergoes uplink transmission power control not only by the serving Node B but also by the non-serving Node B. Therefore, if the non-serving Node B is superior to the serving Node B in terms of uplink channel conditions, the UE may follow a TPC of the non-serving Node B. Because the transmission power of the UE is controlled based on the non-serving Node B, the UE status information can be received at the serving Node B at a very high error rate. In this case, the serving Node B can barely detect the UE status information. In the conventional E-DCH technology, the serving Node B, although it has a high authority, performs scheduling using incorrect UE status information, deteriorating scheduling performance.